This invention relates to an optical disk comprising a plurality of information recording layers and an optical information recording/reproducing apparatus for recording/reproducing information on/from the optical disk.
Conventionally, optical disks have been used as information recording media with a large capacity and a random access capability. And, in recent years, optical disks have been strongly demanded to have larger capacities, since information amount handled in information-associated devices is increasing more and more, and necessity of recording larger capacities of information such as digital dynamic picture images has arised as the information has been presented in multimedia.
To meet such demands, a technology has been proposed lately which enabled information to be read out from each of a plurality of information recording layers provided on one side of an optical disk by changing the focal position of a light for producing information.
As well known, optical disks are classified into three types according to functions; read-only-memory type, write-once type, and erasable type. Each type has its own characteristics and is used differently from others. The erasable type optical disk is excellent in versatility and can be used for writing and reading out dynamic image information, so it is highly expected to be provided with a large capacity. Consequently, one proposal to realize a large capacity erasable optical disk is to provide a plurality of erasable information recording layers on one side of the optical disk. Such a configuration of the disk, however, had a difficulty in being practically used because of a problem to be explained below.
FIG. 1 is a cross sectional view of an erasable optical disk provided with two information recording layers on one side, which is a typical design example of such an optical disk. The optical disk 100 in such a design example is provided with the first information recording part L.sub.1, which is partially reflecting; a spacer layer 105; the second information recording part L.sub.2 ; and a reflective film 109 formed sequentially on one side of the substrate 101. The first information recording part L.sub.1 comprises a transparent dielectric film 102 made of SiN; an information recording layer 103 made of TbFeCo, which is a typical rare earth-transition metal magnetic material; and a transparent dielectric film 104 made of SiN, which are formed sequentially from the substrate 101. In the same manner, the second information recording part L.sub.2 also comprises a transparent dielectric film 106 made of SiN; an information recording layer 107 made of TbFeCo; and a transparent dielectric film 108, made of SiN formed sequentially from the spacer layer 105. The substrate 101 is made of, for example, polycarbonate and the spacer layer 105 is made of, for example, ultraviolet-curing resin. The reflective film 109 is made of, for example, Al.
In the design example shown in FIG. 1, let the refractive index n of SiN be 2.0, the complex index of refraction n-ik of TbFeCo be 3.6-i.multidot.4.1, the wave length of the information reproducing light be 690 nm, and the intensity reflectivity of the second information recording part L.sub.2 be 30%. In this case, the respective relationships between the thickness d (nm) of the information recording layer 103 and each of the intensity reflectivity R.sub.1 of the first information recording part L.sub.1, the final intensity reflectivity R.sub.2 of the second information recording part L.sub.2, which means the ratio of the intensity of the returned light from the second information recording part L.sub.2 to the intensity of the incident light to the first information recording part L.sub.1, and the light absorptivity A of the first information recording part L.sub.1 is as shown in FIG. 2.
In order to stabilize the information reproducing operation in an apparatus for reproducing information from an optical disk having two information recording layers as shown in FIG. 1, both the intensity reflectivity R.sub.1 of the first information recording part L.sub.1 and the final intensity reflectance R.sub.2 of the second information recording part L.sub.2 must become almost equal. And, R.sub.1 and R.sub.2 become almost equal under the above condition when the thickness d of the information recording layer 103 of the first information recording part L.sub.1 is about 6 nm. At this time, both R.sub.1 and R.sub.2 become about 8%. However, it is very difficult to carry out a mass production of such an information recording layer 103 while it is kept accurately to a uniform thickness of about 6 nm. Because the light absorptivity in the information recording layer 103 is too large, the thickness d of the information recording layer 103 must be reduced in such way.
Because of such a problem, it was actually impossible to realize a multi-layer erasable optical disk enabling information to be overwritten at least on the information recording layer provided on the light incident side.